A programmable controller or PLC (“Programmable Logical Controller”) is an automatic control facility capable of driving, controlling and/or monitoring one or more processes, in particular in the field of industrial control rigs, construction or electrical distribution.
Of generally modular design, a PLC programmable controller is composed of various modules which inter-communicate through a transmission bus, generally called a “backplane” bus. The modules are fixed mechanically in a rack, which comprises a printed circuit which also supports the backplane bus as well as the connection elements intended to cooperate with connectors generally present on the rear part of the modules so as to effect the necessary link between the modules and the bus. The number of modules depends of course on the size and the type of process to be automated.
Typically, a programmable controller can comprise:                a power supply module providing the various voltages to the other modules through the backplane bus.        a central unit module UC which comprises embedded software (“firmware”) integrating a real-time operating system OS, and an application program, or user program, containing the instructions to be executed by the embedded software to perform the desired control operations. The UC module also generally comprises a connection on the front face to programming tools of personal computer PC type.        input/output I/O modules of various types as a function of the process to be controlled, such as digital I/Os or analogue TORs for counting, etc. These I/O modules are linked to sensors and actuators participating in the automated management of the process.        one or more modules for communicating with communication networks (Ethernet, CAN, etc.) or man-machine interfaces (screen, keyboard, etc.).        
By way of example, an input/output module can comprise between 1 to 32 I/O pathways, a PLC controller may be capable depending on the model of managing several hundred I/O pathways. If required, several racks are therefore connected together in one and the same PLC. Thus, as a function of the application and the process to be automated, a PLC controller can comprise a large number of modules. It is the user of the PLC controller who therefore decides on the number and positioning of the modules in a rack, as a function of his/her application.
Parallel backplane transmission buses do exist, but henceforth, the backplane transmission bus is often a serial bus. Generally, a serial bus comprises several bidirectional transmission lines and is of the multipoint type in the sense that the bidirectional lines pass through all the connection elements and connectors associated with the various modules connected to the bus.
The equivalent impedance of each line of the backplane bus (line+module connectors+input capacitance of the modules) varies enormously as a function of the number of connected modules and their respective location in the rack, rendering the dimensioning of the bus signals difficult or indeed impossible (=mismatch of the signals). The dimensioning of each multipoint line of a backplane bus, that is to say chiefly the value of the characteristic impedance Z0 adopted for the line as well as its matching at each of the ends of the lines, in fact depends on the presence or otherwise of the modules on the backplane. For example, the more significant the number of modules connected to the backplane bus, the lower the effective characteristic impedance Z0eff.
Now, as has just been seen, it is the user who as a function of his/her application fixes the number and also the location of the modules connected in a rack. It naturally follows that optimal dimensioning cannot be obtained in a systematic manner, thus giving rise to a risk of high consumption due to the low equivalent impedance of the line and a risk of mismatching of the signals, with the additional consequence that a mismatched signal causes significant electromagnetic radiation and generates more harmonics.
This instability phenomenon is all the more pronounced the lower the voltage chosen for the bus signals (for example a voltage of 3.3 V instead of a customary voltage of 5 V) with the aim of consuming less energy (so-called “low power” technology). Dimensioning has shown that the traditional approach of multipoint/multiconnector lines is not suited to this “low power” technology.